What Causes Palpitations And Tachycardia During Iron Transfusion?

I had an iron transfusion last week due to an iron level of 12, iron stores of 2% whatever that means and a TIBC of 489. My H/H was 9.2 and 29, not bad for me. During the transfusion, I began to have palpitations and tachycardia which led them to administer decadron and iv benadryl. What would have caused me to react? I have had 6 iron transfusions in the past...what will my other options be in the future? I can not absorb oral iron.